1. Field of Invention
This invention relates to a method for producing a liquid toner or developer composition, more particularly a liquid toner or developer composition comprising polyester resin particles. The invention also relates to liquid toner or developer compositions made by such a process, and the use of such liquid toner or developer compositions in electrostatographic imaging processes.
2. Description of Related Art
A latent electrostatic image can be developed with toner particles dispersed in an insulating non-polar liquid. Such dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a beam of radiant energy. Other methods are also known for forming latent electrostatic images such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. After the latent image has been formed, it is developed by colored toner particles dispersed in a non-polar liquid. The image may then be transferred to a substrate or receiver sheet, such as paper or transparency.
Insufficient particle charge can result in poor image quality and also can result in poor transfer of the liquid developer or solids thereof to paper or other final substrates. Poor transfer can, for example, result in poor solid area coverage if, for example, insufficient toner is transferred to the final substrate, and can also cause image defects such as smears and hollowed fine features. Conversely, overcharging the toner particles can result in low reflective optical density images or poor color richness or chroma since only a few very highly charged particles can discharge all the charge on the dielectric receptor, causing too little toner to be deposited.
Useful liquid toners comprise thermoplastic resin toner particles and a dispersant non-polar liquid. Generally, a suitable colorant, such as a dye or pigment, is present in the toner particles. The colored toner particles are dispersed in a non-polar liquid which generally has a high volume resistivity in excess of 10.sup.9 .OMEGA.-cm, a low dielectric constant, for example below 3.0, and a high vapor pressure. Generally, the toner particles have an average particle size (by area) or diameter of less than 7 microns as measured with a Horiba CAPA 700 Particle Sizer.
Various methods are known in the art for producing liquid toners and developers. The conventional process for producing such liquid toners involves a two-step batch process, wherein the first step involves a hot step where the non-polar liquid and molten resin are melt mixed with pigment and other additives, and the second step involves a cold step where the molten contents are cooled to cause solidification and formation of particles in the non-polar liquid.
For example, one process of making such liquid toners is described in U.S. Pat. No. 4,760,009, which describes such a two step process. According to the patent, the process comprises a first step of dispersing a thermoplastic resin, a non-polar liquid having a Kauri-butanol value of less than 30, and optionally a colorant at an elevated temperature in a vessel by means of moving particulate media. The temperature in the vessel is maintained at a temperature sufficient to plasticize and liquefy the resin and below that at which the non-polar liquid boils and the resin and/or colorant decomposes. In a second step, the dispersion is cooled to permit precipitation of the resin out of the dispersant. The particulate media is maintained in continuous movement during and subsequent to the cooling. Both steps are carried out in a suitable vessel, such as an attritor, a heated ball mill, or a heated vibratory mill, equipped with particulate media for dispersing and grinding. The result is described as toner particles having an average by area particle size of less than 10 .mu.m and a plurality of fibers. Following the second step, the dispersion of toner particles can be separated from the particulate media.
The patent describes that useful thermoplastic resins or polymers that are able to form fibers and that can be used in the production method include ethylene vinyl acetate copolymers, copolymers of ethylene and an .alpha., .beta.-ethylenically unsaturated acid, copolymers of ethylene/acrylic or methacrylic acid/alkyl (C1 to C5) ester of methacrylic or acrylic acid, polyethylene, isotactic polypropylene (crystalline), ethylene ethyl acrylate series, and ethylene vinyl acetate resins. The patent does not describe use of the disclosed method to produce polyester-based liquid toners or developers.
In fact, the conventional two-step process such as disclosed in U.S. Pat. No. 4,760,009 cannot be used to form acceptable liquid toners or developers from polyester resins. In particular, polyester resins tend not to form a continuous phase with the non-polar liquids such as are used in the hot step. Furthermore, polyesters cannot generally be used in the conventional two-step process because the viscosity of polyester resins at the high temperatures used in the process are too high for adequate dispersion of the colorant and other additives in the resin.
Other methods for forming liquid toners, which are generally variants of the above process, are described, for example, in U.S. Pat. Nos. 5,604,075, 5,688,624, 5,783,349 and 5,866,292. For example, U.S. Pat. No. 5,604,075 describes a process for the preparation of liquid developers with reduced fines, which comprises heating a liquid developer comprised of thermoplastic resin, pigment, charge adjuvant, liquid hydrocarbon, and optional charge director. The heating is accomplished at about 5.degree. C. below the melting point of the thermoplastic resin, which heating enables the fines comprised of the developer components, and of a size diameter of from about 0.1 to about 0.4 micron to be reduced. Although this patent describes that polyester can be used as a suitable thermoplastic toner resin for the liquid developer, improved dispersion of the various toner components is desired to provide improved image development results.
Thus, a need continues to exist in the art for improved liquid toners and developers that provide high print quality, and for methods for producing such liquid toners and developers. This need is particularly evident for producing liquid toners and developers that are based on polyester as the thermoplastic resin.